1. Field of the Invention
This invention relates generally to standard-cell layout designs, and more particularly, to a dynamic system and method for adjustment of the placement of layout objects, such as the cells and etch dummies used with standard-cell layout designs.
2. Description of the Related Art
The feature size of modern standard cell-based layout designs of integrated circuits has reached nanometer scale. For example, the feature size of leading optical lithographic processes is significantly smaller than that of the wavelength of the light used, resulting in a “sub-wavelength lithography” regime. This requires advanced photomasks that embody one or more resolution enhancement techniques (RETs) such as optical proximity correction (OPC), phase shift masks (PSM), off-axis illumination (OAI), and the insertion of sub-resolution assist features (SRAFs). These techniques achieve enhanced critical dimension (CD) control in photo and etch processes. Further, the resolution of the lithographic process is also improved.
A standard-cell layout comprises a plurality of features. This plurality of features can include a plurality of polysilicon lines, active-layer shapes and added features. The plurality of polysilicon lines and active-layer shapes form a plurality of cells in the standard-cell layout. A polysilicon line can be a gate polysilicon shape, a field polysilicon shape and a wiring polysilicon shape. A gate polysilicon shape is formed with an overlapping of a polysilicon line and an active-layer shape. Extra added features include SRAFs and etch dummies. SRAFs typically are scattering bars which are extremely narrow lines, placed adjacent to primary patterns, that do not actually print on the wafer but affect the pattern of light passing through a photo mask on which they are printed. Traditionally, the features are arranged in a horizontal plane in a plurality of rows. The features are oriented vertically in the plurality of rows. A center-to-center distance between two features is termed as a pitch.
With OAI, the illumination is made to fall on the mask at an oblique angle. This angle is chosen to enhance the photolithographic characteristics of the most common pitches in the standard-cell layout. When off-axis illumination is optimized for one pitch (usually the minimum or the most commonly used pitch in the design), there will always be other pitches for which angle of illumination and angle of diffraction together lead to a poor lithographic response and hence a small depth of focus/process window. These pitches are known as forbidden pitches as it is best to avoid them in the layout. Typically they are defined as pitches for which CD has more than 10% error at the worst-case defocus. Avoiding forbidden pitches is a major task involved in designing a standard-cell layout.
SRAFs, as noted above, are dummy geometries that are inserted as a part of an RET flow, to make the isolated pitches “appear” dense, thereby improving the printability of the otherwise forbidden pitches. The correct placement of SRAFs is a major concern involved in the RET of layouts. Incorrect placement of SRAFs can result in unwanted printing of SRAFs on the wafer. It is necessary to maintain a certain minimum spacing between SRAFs and polysilicon shapes as well as between pairs of SRAFs. At the same time, large spacing can make the SRAFs less effective in helping with printability.
Another method of enhancing CD control during photolithography and the etching process is to insert etch dummies during the preparation of standard-cell layouts. In etch processes, different consumptions of etchants with different pattern density lead to etch skew between dense and isolated patterns. Typically, all available etchants in areas with low density are consumed rapidly, and thus the etch rate drops off significantly. To reduce this etch skew, etch dummies are inserted adjacent to the primary pattern with specific spacing. Furthermore, etch dummies are placed outside of active-layer regions. Therefore, etch dummies require correct placement to make printability of resist and etch processes better. However, the insertion of etch dummies and SRAFs together requires precise spacing. Examples of such spacing include the spacing between etch dummies, between polysilicon shapes and SRAFs, and between active-layer shapes and etch dummies. Incorrect etch dummy placement does not allow SRAF insertion in forbidden pitches even when there is enough space to insert multiple SRAFs before etch dummy insertion. Thus, forbidden pitches, resist CD and etch CD degradations occur due to incorrect spacing between various features in a standard-cell layout, including cells, etch dummies, and SRAFs.
There are a number of methods for the reduction of forbidden pitches and resolution enhancement. Some of these techniques are based on the control of certain optical conditions, such as a numerical aperture (NA) and an illuminator aperture shape for OAI. Neglecting SRAFs during layout preparation requires precise sizing of SRAFs and the proper adjustment of the exposure dose. This results in an increase in the complexity of mask inspection, and possibly to CD degradation.
Therefore, there is a need for a method and system that can place layout objects and SRAFs correctly in a standard-cell layout to reduce forbidden pitches. The method and system should result in enhanced CD control in photo and etch processes. Furthermore, the running time of an OPC should not increase substantially.